The present invention relates to a casting apparatus for manufacturing dental prostheses, such as inlay, crown, base, implant, and upper parts of implants, from precious metals or non-precious metals.
A loss of some or all of the teeth as a result of caries (tooth decay), periodontal disease or the like, causes not only functional declination in speech and chewing and/or a change in the facial appearance but also a bad influence on the health of the whole body. It is therefore important to undergo treatment as soon as possible in order to restore the missing tooth (or teeth). According to one of the known restoration methods, a metallic casting is put in place of the missing part of the teeth. Dental prostheses for restoring missing parts of teeth, however, cannot be mass-produced because the teeth shape is unique for each patient and further the size and shape of the missing part differs depending on the case. Therefore, it is necessary to manufacture prosthesis having a particular shape depending on the case of each patient. Also, the prosthesis must be manufactured with a high degree of accuracy to provide a correct occlusion. Thus, in the field of dental casting, the lost wax process, which is known for providing a high degree of accuracy of casting, is generally used for obtaining castings that meet the above demands.
FIG. 19 is a flow chart showing the process of dental casting according to the lost wax process. Referring to FIG. 19, the steps of manufacturing prosthesis used for dental treatment are described. First, a dentist takes a negative impression model of the mouth and teeth around the object part of a patient (Step S1). A dental technician pours modeling material, such as gypsum, into the negative impression, and solidifies the material to produce a positive model (Step S2). The dental technician forms a desired type of casting model, such as an inlay or crown, using wax or resin for the positive model (Step S3). A sprue wire for forming a sprue runner is attached to an appropriate part of the casting model with wax or the like (Step S4). After that, the casting model is detached from the positive model, and the free end of the sprue wire is pushed into a crucible former made of rubber (Step S5).
FIG. 20 is a front view of a casting model mounted on a commonly used crucible former. The crucible former 90 has a conical base 91 formed at its center, and a hole 92 for inserting a sprue wire 94 is formed on the top of the conical base 91. The hole 92 is filled with softened wax, and the free end of the sprue wire 94 (to which the casting model 93 is attached) is inserted in the soft wax. When the wax solidifies, the casting model 93 is fixed on the top of the conical base 91 with the sprue wire 94.
A metallic cylindrical ring (not shown) is fitted onto the crucible former 90 so that the casting model 93 is surrounded by the ring, and investment material such as gypsum or phosphate is poured into the metallic ring to conceal the casting model 93 (Step S6).
After the investment material is solidified, the crucible former 90 is removed, the sprue wire 94 is pulled out, and the ring is heated to a high temperature. By heating, the wax inside is burned off, leaving a cavity corresponding to the sprue wire 94 and the casting model 93. Thus, a mold is obtained (Step S7).
When the mold is heated to a preset temperature, molten metal is poured into a reservoir at the top of the mold, which is a conical depression having a shape corresponding to the conical base of the crucible former. The molten metal flows into the cavity through the sprue runner. This is so-called pouring work (Step S8). After the poured metal has cooled down and solidified, the mold is broken to take out the casting inside (Step S9). Then, unnecessary parts such as fringe metals along the sprue runner are removed from the casting, and after-treatments such as sanding the surface of the cast are carried out (Step S10). Thus, the prosthesis is completed.
In general, the above-described manufacturing work is carried out by a dental technician. Conventionally, in the work of Step S7, an electrical furnace, called a xe2x80x9cring furnacexe2x80x9d, is used to heat the ring to burn off the wax and to heat the mold to a preset temperature. The pouring work of Step S8 is carried out, for example, using a pressure casting apparatus with an inverting casting chamber. An example of this type of casting apparatus is disclosed in Japanese Unexamined Patent Publication No. 2000-176629.
The above type of casting apparatus includes a chamber rotatable within 180 degrees, in which a crucible and a mold are oppositely positioned so that the top of the crucible and the reservoir of the mold face each other. With this casting apparatus, the pouring work is carried out as outlined below. First, posing the crucible with its open top directed upwards, alloy ingots are put in the crucible, and the crucible is heated to melt the alloy. Next, the mold, having been heated to about 800-900xc2x0 C. (degrees Celsius), is placed above the crucible so that the reservoir is directed downwards. Then, the chamber is closed tightly and evacuated with a vacuum pump. As a result, the pressure in the cavity of the mold decreases. After that, the casting chamber is turned upside-down at a preset timing so that the open top of the crucible is directed downwards. Then, the molten metal in the crucible is poured into the reservoir of the mold. The molten metal closes the sprue gate of the sprue runner, so that the pressure in the cavity of the mold is kept low. After that, the evacuation of the casting chamber is stopped, and pressurized air, inert gas or another kind of gas is supplied to increase the pressure in the chamber. The pressure difference between the chamber and the cavity of the mold forces the molten metal to flow through the sprue runner into the cavity of the mold. Thus, the pouring of the molten metal is completed.
With the above type of casting apparatus, it is necessary to carry out the pouring work immediately after the alloy in the crucible has been melted. This is because keeping the metal in the molten state over an excessive length of time will cause oxidization or other quality changes in the metal, which will make the product defective. Therefore, after the metal has been melted, it is necessary to take out the heated mold from the ring furnace and set it in the casting chamber as soon as possible to start the pouring work as described above. As for the ring furnace, the burning needs to be carried out for one to a few hours at about 700-800xc2x0 C. of temperature. Therefore, it is necessary to put the ring in the furnace to start the burning at an appropriate timing, calculating back from the timing of pouring the molten metal.
Thus, being required to manufacture various forms of prostheses, dental technicians spend much time and labor carrying out the casting work as described above.
Further, the temperatures and time periods for burning the ring and melting the metal must be appropriately determined depending on the selection of investment material and alloy material, because a desired quality of prosthesis cannot be obtained when the settings are inappropriate. Conventionally, however, not a few pieces of prosthesis result in being defective (e.g. missing a part) as a result of inappropriately setting the apparatuses, wrong order of work and/or accidental omission of work. This is inevitable when the casting work is manually carried out as described above.
To solve the above problems, the present invention proposes an apparatus for casting dental prosthesis, which greatly improves the efficiency of the casting work while preventing wrong operations in the pouring work.
Thus the apparatus for casting dental prosthesis according to the present invention includes the following elements:
a) a ring-placing platform on which a cylindrical ring with a casting model of a thermally subliming material concealed inside may be placed;
b) a burning unit including a lifting stage and a furnace for heating the cylindrical ring with the casting model to form a mold, where the furnace covers the top of the lifting stage when the lifting stage is lifted to a preset level;
c) a crucible-placing platform on which a crucible containing a casting material may be placed;
d) a casting unit including a chamber having a cylindrical container rotatable about a horizontal axis and a cover for closing the open top of the container, a cover mechanism for attaching and detaching the cover to and from the top of the container, a heater for supplying heat to the container to melt the casting material in the crucible when the crucible is put in the container and the mold is positioned over the crucible so that the sprue of the mold faces the open top of the crucible, and a chamber driver for rotating the chamber about the horizontal axis after the casting material has been melted;
e) a ring conveyer for holding the ring on the ring-placing platform and conveying the ring onto the lifting stage of the burning unit, for holding the ring after burning and positioning the ring over the crucible contained in the container of the casting unit, and for holding the ring after a casting process and returning the ring to the ring-placing platform;
f) a crucible conveyer for holding the crucible on the crucible-placing platform and conveying the ring into the container of the casting unit, and for taking the crucible out of the container and returning the crucible to the crucible-placing platform; and
g) a controller for controlling the burning unit, casting unit, ring conveyer and crucible conveyer so that heating of the ring for a preset time period, heating of the crucible and casting work are performed according to a preset sequence.
With the apparatus according to the present invention, the work is carried out as follows. First, the operator prepares the cylindrical metallic ring with the casting model of a thermally subliming material (wax, for example) concealed inside, and places the ring on the ring-placing platform. Also, the operator places the crucible containing the casting material (alloy ingots, for example) on the crucible-placing platform.
After that, under the control of the controller, the automatic casting operation is conducted as follows. First, the ring conveyer holds the ring on the ring-placing platform and conveys it onto the lifting stage of the burning unit. Then, the lifting stage moves upwards to the preset level to contain the ring in the furnace. In the furnace, the ring is burned for a preset time period to burn off the thermally subliming material and to burn the investment material. Thus, a mold with a cavity corresponding to the molding model is obtained. At a time point earlier than the time point of the completion of the burning by a preset time period, the crucible conveyer holds the crucible on the crucible-placing platform and puts it in the container of the casting unit. The container is heated to maintain the preset temperature, at which the casting material in the crucible melts.
The ring, being burned in the furnace, is ready for use when the casting material is completely melted, and so the ring conveyer conveys the ring with the mold formed inside from the burning unit to a position over the crucible in the container. After the ring is set, the cover mechanism moves the cover to close the container, whereby the chamber is tightly closed. After that, the chamber driver turns the chamber upside-down about the horizontal axis to make the molten casting material flow into the mold. This pouring work can be preferably performed with a pressure control including the following steps: removing the air from the chamber before turning the chamber to establish a low-pressure in the cavity; and increasing the pressure in the chamber to promote a smooth flow of the casting material into the cavity. After the casting material poured into the cavity has solidified, the chamber driver returns the chamber to the original (normal) position. Then the cover mechanism moves the cover to open the container, the ring conveyer conveys the ring to the ring-placing platform and the crucible conveyer conveys the crucible to the crucible platform.
Thus, the apparatus according to the present invention enables an automated process from the burning of the ring to the completion of the casting. All that the operator has to do is to remove the crucible former from the ring after the solidification of the investment material, to prepare a crucible with alloy ingots contained inside and to set the ring and the crucible at preset places in the apparatus. There is no need for the operator to do the troublesome conventional work, such as taking out the ring from the furnace and setting it to the casting apparatus. Thus, the workload of the operator is greatly reduced, so that the working efficiency is improved. Further, the number of defective products due to an incorrect operation or the fault of the operator is greatly decreased.
In a form of the apparatus according to the present invention, each ring conveyer and crucible conveyer includes an arm having a gripper for holding an object; a rotating mechanism for rotating the arm about a vertical axis; a lifting mechanism for vertically moving the arm; and a moving mechanism for moving the arm along a horizontal linear path, where the moving mechanisms of both conveyers commonly include a guide along which the arms are moved. With this construction, it is also preferable to dispose the ring-placing platform and the burning unit across the guide, and to dispose the crucible-placing platform and the casting unit across the guide. This construction provides an effective use of the plane space by virtue of the symmetrical arrangement of the components across the guide.
In another form of the apparatus according to the present invention, the cover mechanism includes a cover-securing mechanism for allowing vertical attaching/detaching movements of the cover when the chamber is in the normal position, while securing the cover to prevent it from falling off the container when the chamber is turned upside-down. The cover-securing mechanism includes a wheel attached to the upper end of the container. The wheel has teeth formed at least at a part of the outer circumference and a stopper projecting from the inner circumference. Plural projections are formed at preset angular intervals around the horizontal axis at a part where the projections engage with the teeth of the wheel. The cover has a notch that comes to the same position as the stopper when the chamber is in the normal position.
With the above construction, when the cover-driver lowers the cover while the chamber is in the normal position, the stopper of the cover passes the notch of the wheel, so that the cover can reach the level where the cover tightly closes the container. In the pouring work, when the chamber is rotated toward the reversed position, the wheel rotates due to the engagement of the teeth with the projections, causing the stopper to be off the notch and press the cover onto the container. Thus, by the above construction, the cover can be securely locked to close the container without using electrical or some other types of driving power source.